1. Field of the Invention
The present invention relates to electrochemical implantable sensor assemblies for monitoring molecules such as Glucose and Oxygen in subcutaneous tissue.
2. Description of the Prior Art
Apparatus utilizing biosensors as measurement components can directly measure biochemical properties of molecules without complex separation steps or additional reagents. Apparatus with electrochemical sensors obtain such properties by measuring electrical signals generated from electrochemical redox reactions of the molecules.
Implantable sensors have the advantage of directly monitoring molecule properties of medical importance, such as concentration of oxygen, glucose and lactate. Such information can be used to improve the accuracy of diagnosis, or the effectiveness of treatment. A subcutaneous micro-sensor directly placed in the physiological cellular environment where metabolism takes place can provide more accurate and timely information of the physiological state. For example, the change of glucose concentration for a diabetic patient is often unpredictable due to a number of factors such as diet, temperature, emotional states, physical activities, age, and rate of metabolism, etc. While discrete measurements cannot provide enough information of dynamic changes, continuous monitoring can in essence provide information that can dramatically improve diagnoses and treatment of diseases.
Reactions of the tissue to an implanted device typically include two types: the first is the tissue's reaction to a foreign body at the cellular and molecular level, which is the mechanism of physiological reactions against a foreign object; the second is the wound healing reaction process that involves a series of time related tissue regeneration steps. Both processes cause perturbation at the implant-tissue interface, forming barrier layer (scar tissue, for example). Such phenomenon may not be a serious problem for some implants of mechanical function (such as artificial bones, ligament, valves, etc.). But for a sensor intended to measure molecules at the interface, the barrier layer isolates the sensor from the ambient issue environment, thus prevents or impairs the molecule infiltration between the sensor and the tissue fluid. Therefore, the sensor measurement would not reflect true molecule properties of the tissue.
The degree of interaction between implanted sensor and the tissue is usually described by “biocompatibility”. One aspect of biocompatibility is biochemical. It may involve physiological reactions such as immune, inflammatory, and anaphylactic reactions caused by the chemical ingredients, additives, or degradation products of the implant. Another aspect of the biocompatibility is mechanical: the physical characteristics of the implanted object such as surface roughness, shape, size, etc. that could cause various degrees of irritations and damages to the tissue. Furthermore, most of the known implanted sensors require auxiliary devices such as catheters or cannula for implanting. Some even requires an incision or surgery. The extra damage to the tissue by the assisting devices is often major factors affecting the performance of the sensors.
Another important factor is the disturbance to the normal physiological environment caused by an excessive flux of molecules consumed by or released from the sensor measuring process. Since the molecules to be detected participate in physiological processes, the more the reaction consumes/releases, the more serious is the interference to the tissue. A lower rate of molecular consumption per unit area (flux) can always be expected to produce a lower degree of disturbance to the tissue. Most sensor designs feature a very small active sensing area while majority of the implanted parts only serves as the supporting body. Such devices require high flux in a very small area to obtain favorable signal. Thus, the localized high molecular flux can result in strong disturbance to the tissue environment and can cause sensor inaccuracy and instability.
Finally, when there are no conventional equipments readily available, problems often exist when attempt is made to mass-produce complicated micro-biosensors involving biochemical processes. Therefore, a simple design with reliable manufacturability is a key for product quality and reliability.